A convex lens forms an image of an object placed 20cm away from it at a distance of 20cm on the other side of the lens. If the object is moves 5cm toward the lens, the image will be

Options

A. 5cm toward the lens
B. 5cm away from the lens
C. 10cm toward the lens
D. 10cm away from the lens

Answer

d.

Explanation

Clearly, `2f=20cm`
or `f=10cm`
Now, `u=-15cm, v=?`
`f=10cm`
`(1)/(v)-(1)/(-15)=(1)/(10)`
or `(1)/(v)+(1)/(15)=(1)/(10)`
or `(1)/(v)=(1)/(10)-(1)/(15)=(3-2)/(30)=(1)/(30)`
or `v=30cm`
The change in image distance is `(30-20)cm`, i.e., `10cm`.